Interpretive Summary: Greenhouse experiments were conducted with glyphosate resistant (GR) cultivars of wheat, to see if there was an increased risk of soilborne pathogens and root disease. GR varieties were not inherently more susceptible than glyphosate sensitive (GS) varieties. Application of glyphosate to GR cultivars did not increase susceptibility to diseases, but GS cultivars treated with sublethal doses of glyphosate suffered more Pythium disease. Crop volunteers were grown in pots with the wheat, and sprayed out at different times before planting. The longer the crop volunteer was left in place, the more reduced the plant growth. GS- volunteers increased Rhizoctonia and take-all disease on GR wheat, when glyphosate was applied to the volunteer either before, at the time of, or after planting.

Technical Abstract:
Control of grassy weeds in no-till wheat is difficult because the lack of cultivation and few herbicides with limited modes of action. Glyphosate is widely used as a non-selective pre-plant herbicide in wheat production, and transgenic glyphosate-resistant wheat cultivars were developed in the early 2000s, although they were not released. Glyphosate has a synergistic effect with soilborne pathogens and root disease. Pathogen infection and inoculum production increase on plants whose defenses have been reduced because of the blockage of shikimic acid pathway due to the inhibition of 5-enolpyruvoyl shikimate 3-phosphate synthase (EPSPS) by the herbicide. The purpose of this research was to determine whether the use of glyphosate resistant cultivars and glyphosate application in-crop could increase the risk of soilborne pathogens. Growth chamber experiments were conducted with two glyphosate resistant (GR) wheat lines and their corresponding glyphosate sensitive (GS) parents and with four pathogens (Rhizocotnia solani and R. oryzae, Gaeumannomyces graminis var. tritici and Pythium ultimum). Experiments were conducted with different herbicide timings and crop volunteer to mimic management practices with crop volunteers planted prior to seeding, where glyphosate would be applied pre- plant; pre-plant and post-emergence; at the time of planting; and post-emergence. Treatments with no volunteers and no volunteer-no spray served as controls. GR-wheat cultivars were not inherently more susceptible to root pathogens than GS-wheat cultivars. Application of glyphosate to GR wheat cultivars did not increase their susceptibility to root pathogens, although the susceptibility to Pythium ultimum was increased in GS cultivars with sublethal doses of glyphosate. When crop volunteer (winter wheat cv. Eltan) was grown in close proximity to GR-wheat cultivars, the timing of glyphosate application had a profound effect. In general, the longer the crop volunteer was left before spraying out, the greater the competitive effect on the crop plant. The highest activity of Rhizoctonia was in the planting and post-planting treatments with volunteer. Both R. solani and G. graminis var. tritici reduced plant height, number of tillers and root length of the GR varieties in the presence of crop volunteer with glyphosate application at the pre-plant, planting, and post-planting stages, compared to treatments without volunteer. Unexpectedly, Pythium ultimum in combination with glyphosate reduced the competitive effects of GS crop volunteer. In order to minimize the damaging effects of these pathogens, producers may need to apply glyphosate at least two to three weeks before planting glyphosate resistant wheat to decrease the inoculum potential of G. graminis var. tritici and R. solani, as currently advised for glyphosate sensitive cereals.